p2p: Close the channel when onStop is called.

P2P dialog is moved to an indenpendent activity, it will push Settings
to background and trigger onPause(). As P2P dialog is a Dialog Activity,
it won't trigger onStop, but SoftAp and NAN activity will. Moving
channel closing to onStop to adapt to new P2P dialog design.

Bug: 219406778
Test: make RunSettingsRoboTests ROBOTEST_FILTER=WifiP2pSettingsTest
      create a p2p connection between 2 devices with this build.
Change-Id: I2fab5aa1021ec1a993f811c7310079db9d7f03c0

tests/robotests/src/com/android/settings/wifi/p2p/WifiP2pSettingsTest.java

@@ -319,12 +319,18 @@ public class WifiP2pSettingsTest {
         assertThat(mFragment.onCreateDialog(-1 /* id */)).isNull();
     }
 
+    @Test
+    public void onStop_notLastGroupFormed_shouldCloseChannel() {
+        mFragment.onStop();
+
+        assertThat(mFragment.mChannel).isNull();
+    }
+
     @Test
     public void peerDiscovery_whenOnPause_shouldStop() {
         mFragment.onPause();
 
         verify(mWifiP2pManager, times(1)).stopPeerDiscovery(any(), any());
-        assertThat(mFragment.mChannel).isNull();
     }
 
     @Test
@@ -332,7 +338,6 @@ public class WifiP2pSettingsTest {
         mFragment.onPause();
 
         verify(mWifiP2pManager, times(1)).stopPeerDiscovery(any(), any());
-        assertThat(mFragment.mChannel).isNull();
 
         mFragment.onResume();
         assertThat(mFragment.mChannel).isNotNull();